Electric discharge tubes



1956 A. H. w. BECK ET AL 2,775,722

ELECTRIC DISCHARGE TUBES 3 SheetsS'neet 1 Filed April 8, 1955 Inventor A. BECK-T. JACKSON .J. LYTOLLJS Attorney 1956 A. H. w. BECK ETAL 2,775,722

ELEC TRIC DISCHARGE TUBES Filed April 8, 1953 5 Sheets-Sheet 2 J. LYTOLLI ywzz Attorney Inventor A. BECK-T- JACgSON- Dec. 1956 A. H. w. BECK ET AL 2,775,722

ELECTRIC DISCHARGE TUBES Filed April s, 1953, s Sheets-Sheet s Anode Lo Anode 60p Hes/stance in Oh Gas Pressure mm. 9 by) x Gap Length mm.)

Inventor A. BECK-T. JACKSO J. LYTOLLI S A Home y United States Patent ELECTRIC DISCHARGE TUBES Arnold Hugh William Beck, Thomas Meirion Jackson, and John Lytollis, London, England, assignors to International Standard Electric Corporation, New York,

Application April 8, 1953, Serial No. 347,486

Claims priority, application Great Britain April 10, 1952 12 Claims. (Cl. 315-169) The present invention relates to cold cathode gas-filled electric discharge tubes and switching circuits using such tubes and is particularly concerned with the construction and use of such tubes in switching circuits carrying speech currents.

Cold cathode gas-filled electric discharge tubes are frequently used as voltage operated relays and the like in switching circuits. Dilliculties are encountered, however, when it is desired to complete a speech carrying path through a cold cathode tube. Not only is it known that if the cathode-anode discharge path of a cold cathode tube be included in a speech current circuit there is a tendency for noise to be injected into the circuit, but difiiculty has been experienced in reducing the impedance of the cathode-anode path to values much less than 1000 ohms, which means that the insertion loss of the switch is high, particularly if inserted directly in a normal 600 ohms circuit. In addition, inertia efiects of the gaseous ions result in the impedance of such a path appearing as a resistance shunted by an inductance whose value may be of the order of henries.

In the course of experiments to devise a cold cathode electric discharge tube which could be used as a switch to open or close a path in a speech circuit through the discharge tube, the present applicants have found that it a cold cathods tube having two adjacent anodes spaced from a common cathode be used with the anodes connected in parallel for D. C. purposes but in series for speech currents, the impedance of the path between the anodes in the gaseous discharge is very much lower than had previously been obtained with any other arrangement and noise voltages are not troublesome.

From the point of view of circuit use, therefore, the present invention provides a switching circuit comprising an electric discharge tube having a cold cathode and two anodes cooperating therewith, and means for establishing an electric transmission path across the gap between the two anodes by establishing discharges between the said cathodeand the respective anodes. From one constructi-onal aspect the invention provides a cold cathode gasfilled electric discharge tube comprising a cathode and a plurality of anodes cooperating therewith 'so that a glow discharge may pass from the cathode to each anode, the said anodes being disposed relative to one another and to the cathode so that during discharge between the cathode and said anodes an electric transmission path is provided across the gap between the anodes or a pair of said anodes.

A low anode-anode impedance can be obtained if the two anodes are disposed with respect to the cathodes so that they are both at the edge of the cathode dark space. This means however, that for each gap the anode-cathode striking and maintaining voltages are then similar, which in general is not desired. According to a further aspect of the invention, therefore, there is provided an electric discharge device comprising within an envelope containing an ionisable gas, a cold cathode and two anodes disposed with respect to the said cathods so that the mainice taining voltage of each gap between the said cathode and either anode is less than the striking voltage and so disposed with respect to each other that a path across the gap between the two anodes presents a low impedance to speech currents.

From the point of view of obtaining a maximum difference between the striking and maintaining voltages for the anode-cathode gaps, the anodes should be as far distant as possible from the cathods. If the anode-cathode gap is too great, however, not only does the anode-anode impedance, rise, as will be explained later, but noise is introduced so soon as the anode-cathode gap is longer than that appropriate to a limited region of the Faraday dark space. According to a further aspect of the invention, therefore, there is provided a cold cathode gas-filled electric glow discharge tube comprising a cathode and two mutually adjacent anodes each so positioned that during abnormal glow discharge between the cathode and both anodes the said anode is situated between the edge of the cathods dark space and the end of that region of the Faraday dark space in which there is substantially no electron space charge sheath surrounding the anode.

Various possible electrode configurations may be used; for example the cathode may be a flat strip, and the anodes may be adjacent rods or further fiat strips parallel to one another and both parallel to the cathods or at rightangles to it. Adopting a cylindrical geometry, other possibilities are: rod-shaped anodes with a surrounding cathode, or a rod-shaped cathode with surrounding anodes.

With certain methods of use it may be advisable to take steps to eliminate the statistical delay of firing of the anode-cathode gaps, in which case the discharge tube may include means for ejecting photo electrons from the cathode. Although for certain applications a simple diode type of tube is required, for other applications a trigger electrode may be incorporated to provide means for firing the anode-cathode gaps.

Embodiments of the invention will be described with reference to the accompanying drawings in which:

Fig. 1 shows a circuit diagram to illustrate the use of a tube according to the present invention;

Fig. 2 shows a circuit diagram of an arrangement according to the invention utilising a double tube providing switches in a pair of lines balanced to ground;

Fig. 3 shows the construction of an experimental tube according to the invention;

Fig. 4 shows curves illustrating the behaviour of the tube of Fig. 3;

. Figs. 5 to 10 show diagrammatically alternative electrode arrangements to that of Fig. 3;

Fig. 11 shows curves illustrating the efiect of anodecathode gap length on anode-anode impedance, and

Fig. 12 illustrates the use of an auxiliary trigger electrode for initiating the main anode-cathode discharge.

In Fig. 1 there .is shown a cold cathode electric discharge tube 1 having a cathode 2 and a pair of anodes 3 and 4. The cathode 2 is connected .to ground through a resistor 5 and rectifier element 6 connected to have a low impedance when current flows from cathode to ground. Resistors 7 and 8 connect the respective anodes to .a terminal 9 which is connected to the positive pole of a source of D. C. potential not shown. \A speech transformer i10 has one of its secondary terminals connected through D. C. blocking condenser 12 and a load circuit H to anode 3. The other end of the secondary winding is connected through D. C. blocking condenser 16 to 'anode 4. The resistors 5, 7 and 8 and the voltage of the D. C. source connected to terminal 9 is such that when once the gap between either of these anodes and cathode Q has 'been 'fired, the discharge is maintained and covers the whole of the cathode surface but the voltage available is insuflicient to tire the gap initial-1y. For

firing purposes a resistor 14 is connected between cathode .2 and terminal '15, resistor 14 being of high i-rnpedance. A negative pulse applied to terminal 15, when rectifier 6 is not conducting, lowers the cathode potential sufficiently for the tube to fire. The discharge current passing through rectifier 6 lowers the rectifier resistance and renders inoperative any further negative potentials applied to terminal 15. Before the tube is fired, the impedance between anodes 3 and 4 is very high, so that substantially no speech current can be passed from transformer 10 through the load circuit 111. When the tube is fired, however, the impedance between anodes 3 and 4 becomes very low and the circuit to the load I11 is completed. The discharge in the tube may be extinguished by app-lying a negative pulse to terminal 9 to lower the anode voltage below the maintaining voltage of the tube. It is to be understood that the methods of firing and extinguishing the tube are described above merely by way of example and various alternative methods may be used.

When using a tube whose construction will be further described below We find when the cathode glow completely covers the cathode-i. e. the discharge is abnormal that at speech frequencies the path between anodes 3 and -4 presents a substantially resistive impedance which may be between 50 and 80 ohms. Noise is not notice able nor does the anode-anode impedance vary appreciably with frequency in the audio frequency band. Tests at frequencies up to 50 kc./s. have shown that the impedance is substantially resistive and independent of frequency. It is found possible to apply speech voltage between the anodes without objectionable harmonic distortion, even when the amplitude is nearly sufiicient to cut off the D. C. discharge current to one of the anodes during a half cycle. Furthermore, if an excessive surge voltage be applied between the anodes, the current to one of the anodes will be cut off and that to the other will tend to a fixed maximum value independent of the applied voltage. After removal of the excess voltage the anode which was cut off will refire due to ionisation coupling from the discharge to the other anode. If, on the other 'hand, the anode-cathode gap of a diode type of cold cathode tube were used, a voltage surge, if in one direction, may extinguish the discharge altogether, or, if it drives the anode positive, may cause excessive current to pass. With the present invention, the switching tube also functions as a surge limiter and the cathode discharge is not extinguished by the surge.

In Fig. 2 there is shown an arrangement of the invention which is more suitable for use with balanced circuits. A pair of transformers 16 and '17, having centre-tapped secondaries, provide input and output speech connections. The impedance ratio of the transformers is preferably such as to match the high impedance secondary circuits. A double tube 18 having two electrode assemblies similar to those of the tube of Fig. 1 is used with one pair of anodes 19 and 20 connected each to one end of the secondary windings of the respective transformers 16 and 17, the lead .from anode 20 being taken through a rectiher 21 as shown. The other ends of the secondary windings of the transformers are connected to the other pair of anodes 22 and 23, la rectifier '24 being connected as shown in the lead to anode 2 3. Cathodes 2'5 and 26 are connected to ground through respective choke coils 27 and 28 and rectifiers 29 and 30. All the recti'fiers are connected so as to present a low impedance path to anodecathode discharge currents through the tube. High tension supply sufficient to maintain discharge from the anodes of each half of the tube to their respective cathodes is connected to the centre taps 31 and 62 .of the secondaries of transformers 16 and 17 through current limiting resistors 33 and 34. Resistors 35 and 36 of high value are connected between anodes 20 -and.2'3 and respective terminals 37 and 38 to which positive pulses may be applied simultaneously. Negative pulses may be applied simultaneously to cathodes 2-5 and 26 from respective terminals 39 and 40 through high valued resistors 41 and 42. These arrangements are such that the simultaneous presence of positive pulses at terminals 37 and 38 and negative pulses on the corresponding cathode terminals B9 and 40 are required to fire the tube. When this coincidence occurs, the two halves of the tube fire and the rectifiers 21, 24, 29 and 30 then apply low impedance shunts to the pulses so that the latter no longer have any eflect. The discharge gaps cathode 25anode 19 and cathode 26anode 22 are primed lby ionisation coupling from the discharge to the respective adjacent anodes. When both halves of the tube are fired, connections are established between the secondaries of the transformers 16 and 17 and the speech circuit is completed. To extinguish the discharge, and hence to sever the connections between transformers 16 and 17, a negative pulse may be applied to points '31 and 32 to take the anode voltages below the maintaining voltage of the tube. As with Fig. 1, so in this embodiment the firing and extinguishing arrangements are described by way of example only; thus, positive extinguishing pulses may be applied to terminals 39 and 40.

With the above arrangement characteristics similar to those described in connection with Fig. 1 were obtained. Due to the low anode-anode impedance coupled with the comparatively high A. C. impedance between the anodes and their respective cathodes, together with the chokes 27 and 28, the insertion loss of the anode-anode gaps is small, values of 0.1 to 0.4 db being obtained.

Referring now to Fig. 3, one type of experimental tube which has been constructed is similar in appearance to that illustrated, being housed in a conventional minaturc radio valve type of envelope 43. A vertical sheet of mica 44, supported between mica discs 45 and 46, carries a rectangular cathode plate 47 on one side and a similar cathode plate 47 on the other. The cathode plates are mounted spaced away from the supporting sheet 44 so as to avoid trouble due to cathode sputtering. The anodes take the form of rods such as 48 and 49 mounted with their axes lying in a plane parallel to the cathode. The ends of a further pair of anode rods 50 and 51 for the other half of the tube are seen projecting through the disc 45.

With this geometry, low anode-anode impedances have been obtained with pure inert gases, such as helium, argon and neon and with mixtures of gases, notably neonargon and neon-argon-hydrogen. In early experimental tubes with a gas filling of 99% neon and 1% argon at 30 mm. pressure of mercury, anode-cathode separations were chosen to provide a striking voltage to either anode of about 200 volts, the maintaining voltage lying between and volts. A satisfactory tube having helium at a similar pressure had striking voltage of 300 and a maintaining voltage of 200 for each anode.

It will be seen that the difference between striking and maintaining voltages was considerably higher for the helium-filled tube than for the neon-filled tube. Further reference to this matter will be made below.

The spacing between anodes has been found not to be critical, otherwise similar tubes in which the anode-anode separation varied from 5 mm. to 0.5 mm. differing insignificantly in characteristics.

In a tube according to the invention, if the anodes are supplied separately from different sources of potential, it is observed that over a range of potential difference between the anodes the diiference in current flowing to the two anodes is substantially linear. A typical ditferential characteristic is represented by the curves of Fig. 4. To obtain these curves a circuit similar to Fig. 1 was used, an additional D. C. source being substituted for the transformer 10, load 1i. and blocking condensers 12 and 13. Meters were inserted in the lead to each anode and the anode currents plotted against anode-anode potential diiference. The total cathode current was adjusted so that abnormal discharge occurred i. e. the whole of the cathode surface was covered by cathode glow, each anode when at the same potential, passing 12.5 ma. discharge current. As the voltage of one anode was raised, the current to it, plotted on curve A, increased at first linearly and fairly rapidly and then tended to remain constant, the current to the other anode-curve Bbeing reduced in similar manner. Thus, for the tube to which these curves relate, over a range of about :1 volt, a difference in potential applied between the two anodes causes a proportional difference in the discharge current to them, the total discharge current remaining approximately constant. If alternating current is applied, therefore, the tube behaves similarly to a 1:1 transformer coupled to an on-ofi switch and voltage limiting means.

Although best operating conditions are obtained when the cathode load resistance is large, a cathode resistor of 17.5 k and anode resistors of kn being used in the circuit to which the curves of Fig. 4 relate, the degree of balance is not critically dependent on the relative values of anode and cathode D. C. load, a reduction of the cathode load to 5 k9 being permissible.

In Fig. 3 a single pair of anodes is shown cooperating with each cathode. An alternative arrangement is shown in Fig. 5. We have found that the impedance between the anodes can be reduced if four similar anode rods 52 55 respectively, are mounted in a line with one another parallel to the cathode 47, alternate anodes 52, 54 and 53, 55, respectively, being connected together to replace the single anodes 48 and 49 shown in the drawings. Similarly six or more anode rods could be used if desired.

Other types of anode construction are indicated diagrammatically in Figs. 6 to 10, in each of which the electrodes are indicated by the same reference numerals as in the schematic circuit diagram of Fig. 1. Thus in Fig. 6 the cathode 2 comprises a rectangular plate and the anodes 3 and 4 are strips parallel to one another and to the cathode 2.

Under certain conditions of operation, notably pulse conditions, embarrassment may be caused by the statistical delay in firing of the anode-cathode gaps. It is well known that, in the absence of any ionisation between cathode and anode in a cold cathode tube, an anode-cathode voltage higher than that normally necessary to cause breakdown can exist for some time before breakdown occurs; as soon as some random charged particle enters the gap, a breakdown can occur. The charged particle may originate from ionisation due to cosmic rays, for example, or the action of light on the cathode, the delay in firing pending the occurrence of such an event being known in the art as the statistical delay of firing of a gap. The statistical delay may be eliminated by inserting some radioactive substance Within the tube envelope, by subjecting the cathode to light of suitable wavelength, or by providing ions by means of an auxiliary priming discharge or night light. In Fig. 6, two additional electrodes 56 and 57 are indicated between which a night light discharge may be maintained. As it is not desired to reduce the anode-cathode striking voltage but only to reduce the statistical delay of firing of these gaps, it is desirable that no charged particles should be capable of being attracted from the night light discharge to any of the main electrodes. The light from the discharge is sufficient to liberate photo-electrons from the cathode and, therefore, the electrodes 56 and 57 are enclosed in a quartz tube which will allow light of the required short wave length to pass without substantial attenuation while preventing charged particles reaching the other electrodes and so reducing the anode-cathode striking voltage. As shown in Fig. 6 the gap between electrodes 56 and 58 is in register with the space between anodes 3 and 4 and the glow between electrodes 56 and 58 passes through said space to fall upon the surface of cathode 2.

In Fig. 7 the cathode 2 is again represented as a plate, but the anodes 3 and 4 are strips parallel to one another and edge-wise on to the cathode. In this example, in place of the priming discharge electrodes 56 and 57 and the quartz tube 58, a wire loop 59, which can be heated, is placed in the vicinity of the discharge gaps. We have found that if a filament of tungsten wire is heated to about 1400 K., suflicient illumination of the cathode is obtained to eliminate the statistical delay of firing of the anode-tocathode gaps, but there is no significant alteration in the other characteristics of these gaps.

In Figs. 8, 9 and 10, cylindrical constructions-are shown. In Fig. 8 the cathode 2 is a rod surrounded by coaxial cylindrical anodes 3 and 4, spaced apart along the axis of the cathode, while in Fig. 9 the cathode 2 is cylindrical and the anodes 3 and 4 are axial rods inserted at either end so as to leave a small gap between them. In Fig. 10 the cathode 2 is again cylindrical and the anodes 3 and 4 are rods which, in this example, are positioned side-by-side symmetrically disposed within the cathode.

Experimental tubes employing the different arrangements illustrated have been constructed. In the case of the arrangements of Figs. 9 and 10, priming of the anodecathode gaps is diflicult due to the shielding effect of the surrounding cathode; for many applications, however, the statistical delay of firing can be tolerated.

As has been indicated above, the separation between the anodes of any of the arrangements described is not critical. The anode-cathode separation, however, must be chosen with care, particularly when the anode-cathode gaps are to be fired by coincident pulse arrangements. Thus, for use in a coincidence pulse arrangement such as described with reference to Fig. 2, it is desirable that there should be a fairly wide margin between the striking voltage and maintaining voltage for each anode-cathode gap. We have found that, for various gas fillings, if the anode-anode impedance be plotted as ordinates against the product of anode-cathode separation and gas pressure a curve is obtained which, at low values of the product, is fairly flat but which rises rapidly at higher product values.

Examples of such curves are shown in Fig. 11 in which curve A referes to an argon-filled tube with a cathode current of 20 ma., curve B relates to a helium-filled tube at a cathode current of 20 ma., and the curve B relates to the same tube as the curve B but with the cathode current reduced to 10 ma. It will be seen that in all cases the anode-anode gap resistance is fairly constant over part of the range of abscissa, but then increases rapidly. This rapid increase of anode-anode impedance is associated with the anodes being positioned beyond the end of the region of the Faraday dark space in which there is substantially no electron space charge sheath surrounding the anode.

In a glow discharge over a long path there appears a dark space adjacent the cathode, known as the Crookes or cathode dark space, followed by a luminous glow, known as the cathode glow, and a second dark space, known as the Faraday dark space, which is succeeded by the luminous region referred to as the positive column. In the Faraday dark space the flow of current is almost entirely electronic, there being very few positive ions present, whereas in the positive column approximately equal numbers of positive ions and electrons flow in opposite directions forming a gas plasma. In the positive column plasma oscillations occur and any transmission path involving the positive column is, therefore, subject to noise. It has, therefore, previously been suggested that the anode of a glow discharge tube for use in telephone circuits and the like should be positioned within the Faraday dark space to avoid noise. We have found, however, that if the anode is near the end of the Faraday dark space, the electrons of the discharge, which are being accelerated towards the anode, may acquire sufficient kinetic energy to liberate secondary electrons from the material of the anode. These secondary electrons establish an electronic space charge sheath around the anode, under which conditions, in an arrangement such as that of Fig. 1, the transmission path becomes noisy and the anode-anode impedance rises. For a considerable part of the Faraday dark space, however, the formation of an electronic space charge sheath around an anode is negligible and noise in the anode-anode transmission path is low.

In order to have as wide a margin as possible between anode-cathode striking and maintaining voltages in embodiments of the invention the anode-cathode gap length should obviously be as large as possible. Reference to Fig. 11, however, shows that, quite apart from the question of noise, the rise of the anode-anode impedance dictates that the product of gas pressure and anode-cathode gap length should be below values which would correspond to the steep skirts of the curves shown and the formation of an electronic space charge sheath adjacent the anodes. It should be mentioned that in the case of neon we have not found such sudden increases of anodeanode impedance as in the case of helium and argon. On the other hand, as has been mentioned earlier on, for a neon filled tube it is difficult to obtain any considerable margin between maintaining and striking voltages.

For telephone switching purposes, our experiments to date indicate that an electrode structure similar to that illustrated in Fig. 6 is the most generally useful, except that, in place of the priming gap arrangements there shown, a heated tungsten wire, as the wire 59 in Fig. 7, is substituted. In such practical embodiments we have used an anode-cathode gap of 2 mm. in a helium atmosphere at a pressure of 70 mm. of mercury, obtaining a minimum breakdown voltage of 240 v., a maintaining voltage of 115 v. and an anode-anode series resistance of from 80 to 110 ohms.

In the arrangements so far described a simple diode type of anode-cathode gap has been used. For many purposes in the telephone art this is preferred. There are, however, applications in which a trigger electrode can usefully be employed to fire the cathode-anode gaps. Such an arrangement is illustrated in Fig. 12, in which, in addition to the cathode 2 and anodes 3 and 4, a trigger electrode 60 is employed. The gaps between the trigger electrode 60 and cathode 2 may have either a lower or a higher breakdown potential than that of either of the gaps between cathode 2 and anodes 3 and 4. With the first, the more usual arrangement in a trigger tube, the cathode-anode separation must be greater than the length of the cathode dark space, so that the anodeanode impedance is not the smallest possible. Alternatively, the anode-cathode gap may be reduced to the length at which a maximum valued signal surge, imposed upon the D. C. anode-cathode voltage in the absence of discharge, is just insufiicient to cause breakdown. This value of gap-length will then provide the lowest prac tical value of anode-anode impedance. The triggercathode gap length may then be chosen to provide .a convenient value of trigger gap breakdown potential which may be greater than that of the anode-cathode gaps.

In addition to the other electrodes, the electrodes 61 and 62 form an auxiliary discharge gap, this gap being shielded by means, such, for example, as a quartz partition, indicated at 63, so as only to eliminate the statistical delay of firing of the cathode to trigger gap but not to affect the other electrical characteristics of the tube.

in Fig. 12 the priming gap is shown energised by a D. C. source 6 5 connected across the gap electrodes through a currentdimitfng resistor 65. The anodes 3- and 4 are inter-connected by the secondary winding of transformer 66 and are each connected to a source of high tension through individual resistors 67 and 68, respectively, and a common anode resistor 69. The trigger electrode 60 is shown connected through a resistor '76 to terminal '73, while an additional connection to the anode circuit is provided through D. C. blocking capacitor 72 to terminal 73. The cathode 2 is connected to ground 8 through a high impedance choke 74. A positive pulse applied to terminal 7]. fires both anode-cathode gaps and so establishes the speech path between the anodes 3 and 4, so that a low impedance is presented to any circuit terminated by the primary winding of transformer 66.

The tube may be extinguished by means of a negative pulse applied to terminal 72, after which the impedance across the primary terminals of transformer 66 becomes high.

While the principles of the invention have been describcd above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What we claim is:

l. A switching circuit for switching a signal source to a load comprising an electric discharge tube having a cold cathode and two anodes cooperating therewith, means for establishing a transmission path across the gap between the two anodes by establishing discharges between the said cathode and the respective anodes, and means coupling said transmission path in series between said source and load.

2. A cold cathode gas-filled electric discharge tube arrangement for switching between a signal source and its load comprising a cathode and a plurality of anodes cooperating therewith so that a glow discharge may pass from the cathode to each anode, the said anodes being so disposed relative to one another and to the cathode that during discharge between the cathode and the said anodes an electric current transmission path is provided across the gap between each pair of the said anodes, and means coupling the anodes of each pair and their gap in series with each other and in series between the source and load.

3. An electric discharge device arrangement for switching between a signal source and its load comprising, within an envelope containing an ionisable gas, a cold cathode and two anodes disposed with respect to the said cathode so that the maintaining voltage of the gap between the said cathode and either anode is less than the striking voltage and so disposed with respect to each other that, during discharge between the cathode and the said anodes, a path for signals is provided of low impedance at least for signals in the voice-frequency range, and means coupling said anodes in series between said source and said load.

4. A cold cathode gas-filled electric discharge tube arrangement for switching a signal source to its load comprising a cathode and two mutually adjacent anodes each so positioned that during abnormal glow discharge between the cathode and both anodes the said anodes are situated between the edge of the cathode dark space and the end of that region of the Faraday dark space in which there is substantially no electron space charge sheath surrounding the anode, and means coupling said anodes in series between the source and load.

5. A cold cathode gas filled electric glow discharge tube arrangement for switching a signal source to a load comprising a cathode and a pair of mutually adjacent anodes characterised in this, that the product of the pressure of the gas filling and the gap length from the cathode to either said anode is such that, for a given glow discharge current from the said cathode, the said product lies within the range of values for which, at that said current, the resistance of the gap between the said anodes is near its minimum value and varies slowly with change of the said product when compared to its rate of variation outside the said range, and means coupling said anodes in series between the source and load.

6. A tube arrangement according to claim 2 further comprising a planar cathode and a pair of planar anodes parallel to one another and opposed to and parallel with the said. cathode.

7. A tube arrangement according to claim 2 in which said cathode and anodes comprise a planar cathode and a pair of planar anodes parallel to one another and orthogonally opposing the said cathode.

8. A tube arrangement according to claim 2 in which the cathode and anodes comprise a planar cathode and a plurality of rod-shaped anodes mounted in a plane parallel to the said cathode.

9. A tube arrangement according to claim 2 in which the said cathode comprises a rod and the said anodes comprise a pair of cylinders mounted end to end coaxially about the said cathode.

10. A discharge tube according to claim 2 in which said cathode and anodes comprise a cylindrical cathode and a pair of anode rods projecting from either end within the enclosure of the said cathode along the axis thereof.

11. A tube arrangement according to claim 2 in which an auxiliary discharge gap is positioned to illuminate the 10 said cathode and is so mounted that charged particles from the said auxiliary discharge gap are shielded from the said cathode and anodes.

12. A tube arrangement according to claim 2 comprising cathode illuminating means adapted on being heated by the passage of current therethrough to illuminate the said cathode.

References Cited in the file of this patent UNITED STATES PATENTS Williams et a1 Apr. 15, 1952 

